Mmmm I'm not sure that Castiel said that he knows what is gonna happen in the future... he only said that destiny can't be changed. So, Dean could'nt change the path of his parents, because it's already happened in the past. But I don't remember him saying that he knows (or can go in) the future though,... Maybe I'm wrong, I'm a french canadian but i hate watching tv shows with the french traduction, maybe i missed something. (Sorry if my english is bad!)